Add `layer` argument to `filter_rank_genes_groups()`

[lazappi]

What kind of feature would you like to request?

Additional function parameters / changed functionality / changed defaults?

Please describe your wishes

The filter_rank_genes_groups() function is currently missing a layer parameter and defaults to using X. This is inconsistent with rank_genes_groups() and most other functions. Practically, in most cases pre-calculated values are used anyway but it would be nice to have this as a consistent option.

[flying-sheep]

Good point! Ref paths are coming, but I might release this in 1.13 anyway.